Safety and efficiency in eliciting an effective host immune response are the two major criteria in developing recombinant viral vaccines against wild-type viral infections. An ideal recombinant viral vaccine should not only be replication-defective and capable of eliciting a broad protective immune response, but should also encode a safety mechanism that can inhibit de novo replication of naturally occurring wild-type virus if encountered within the same cells in the host. Herpes simplex virus (HSV) infections can cause significant clinical problems and even death in individuals who are immunodeficient or suffering from disorders of skin integrity. Approximately 63% of adults in the U.S are infected by HSV. Currently, there is no effective medication that can prevent primary HSV infections nor decrease the incidence of recurrences. Thus, there is a great need for developing a safe and effective HSV vaccine. We recently generated a prototype HSV recombinant virus, CJ83193, which is capable of inhibiting its own replication as well as that of wild-type HSV-1 and HSV-2. On the basis that CJ83193 can serve as a prophylactic vaccine against HSV-1 primary infection in mice at levels comparable to that of wild-type HSV-1 and that CJ83193 has shown great potential as a therapeutic vaccine to reduce recurrent infection, the overall objective of this research program is to further characterize and develop a new class of CJ83193-derived HSV recombinants whose safety and efficacy as vaccines against HSV infection can be greatly enhanced. Several novel approaches will be employed and developed in this grant application. Specific Aim 1 will focus on the development of CJ83193-1ike HSV recombinants with increased safety features and efficacy in blocking wild-type HSV viral DNA replication. The potential of these HSV recombinants as effective vaccines against HSV infection in mouse models will be evaluated in Specific Aim 2. Given that CJ83193 expresses a very low level of HSV-1 major antigen glycoprotein gD and, like other well-known replication-defective HSV recombinant viral vaccines, is incapable of expressing gC, another HSV-encoded major glycoprotein and a major target for CD4+ CTL, the goal of Specific Aim 3 is to enhance the efficacy of CJ83193-derived recombinant HSV vaccines with a gene switch that could lead to high-level expression of gD and/or gC in the vaccinated host. Recall that B7 costimulation plays a central role in the generation of anti-HSV-specific cellular immunity and T-cell-dependent antibody responses as well as infection of immature DC by HSV-1 leading to down-regulation of the expression of B7-1 and B7-2. Specific Aim 4 will focus on investigating the influence of de novo expression of B7-1 and B7-2 by HSV-1 replication-defective vaccine virus on the augmentation of host immunity against HSV infection. [unreadable] [unreadable]